1. Technical Field
The present invention relates to a two-wire transmitter which is connected to an external circuit by two transmission lines and which outputs a prescribed current signal to the external circuit while using the external circuit as a power source.
2. Related Art
A two-wire transmitter is a device which is connected to an external circuit by two transmission lines and which converts prescribed information (a physical quantity) acquired from a sensor or the like into a current signal and outputs the current signal to the external circuit while using the external circuit as a power source. Two-wire transmitters are used widely as field devices such as a differential pressure/pressure transmitter and a temperature transmitter in individual plants because they do not require a dedicated power wiring and can be installed at a low cost. When used as a field device, a two-wire transmitter converts a physical quantity into a DC current signal of 4 to 20 mA (world standard of a field device signal) and sends it to an external circuit.
Japanese Patent Document JP-A-2007-66035 describes a current monitoring device which is a field device and employs a two-wire transmission scheme that does not require a power wiring as in two-wire transmitters. The current monitoring device described in JP-A-2007-66035 is equipped with a power voltage generator (shunt regulator) which performs a constant voltage control to stabilize circuit operation. The shunt regulator described in JP-A-2007-66035 performs a control so that the potential of a VSUP line (a circuit voltage of the current monitoring device) becomes equal to a reference potential VR. The reference potential VR is fixed by means of a resistor and a reference voltage source VREF such as a Zener diode. This type of shunt regulator is also used in general two-wire transmitters.
Incidentally, in recent years, two-wire transmitters have come to be required to be increased further in circuit operation speed, enhanced in insulation performance to increase the sensor S/N ratio, and added with such functions as self-diagnosis. To satisfy such requirements, it is necessary to secure more consumable power in the circuit.
However, in conventional two-wire transmitters, as described later, it is difficult to attain both of securing of more consumable power in the circuit and stabilization of circuit operation by the shunt regulator.
In a two-wire transmitter used as a field device, the current (supply current) that is supplied from the external circuit is varied as the output current signal varies (4 to 20 mA). On the other hand, the power voltage of the external circuit, which corresponds to the circuit voltage of the two-wire transmitter plus voltage drops across a feedback resistor and a detection resistor through which the supply current flows, is approximately constant.
However, as the output current of the two-wire transmitter increases and the supply current increases accordingly, the voltage drops across the feedback resistor and the detection resistor are increased and the securable circuit voltage is lowered. The circuit voltage of the two-wire transmitter is minimized when the output current is equal to the maximum value (20 mA). From another point of view, at least a circuit voltage corresponding to the maximum output current can always be secured irrespective of the output current.
In view of the above, in conventional two-wire transmitters, the shunt regulator fixes the circuit voltage in a low voltage range around the power source voltage minus its own maximum voltage drop. With this measure, although the circuit operation is stabilized, because of the low circuit voltage only a small consumable power is secured when the output current is small (e.g., 4 mA) and hence the supply current is small.